1. Field of the Invention
The present invention relates to a frame structure for a saddle-ridden-type vehicle such as a motorcycle, and more specifically, to a frame structure of an aluminum twin spar frame.
2. Description of the Related Art
In a related-art frame structure for a motorcycle, a front end portion of each tank rail is welded to a head frame having a head pipe. In the head frame disclosed in Patent Document 1, a rectangular-tube-shaped joint portion protrudes from a side surface of a head pipe, and a front end portion of a rectangular-tube-shaped tank rail is fitted with the joint portion in an overlap manner. The joint portion of the head frame and the front end portion of the tank rail are subjected to fillet welding over whole circumference thereof, to improve joint strength between the head frame and the tank rail.
Patent Document 1: Japanese Patent Application Publication No. S62-034879A
In the motorcycle, however, a bending load is applied to the welded portion between the head frame and the tank rail due to push-up from a road surface or a brake force of a brake. In this instance, stress concentration is easily produced on an upper surface side of the welded portion between the head frame and the tank rail. Further, in the frame structure disclosed in Patent Document 1, since the joint portion of the head frame and the front end portion of the tank rail are formed in the rectangular-tube-shape, it is not possible to continuously weld them over a corner portion having a high curvature. For this reason, an end portion of a weld bead is formed at a corner portion of the upper surface side of the welded portion, and thus a load is likely to be applied to the welded portion.